| The most common method to get an idea about the | | | | ions increase, pH decreases. The indicator equilibrium |
| pH of solution is to use an acid base indicator. An | | | | shifts to left, if In- ions decrease. Below pH 8.2 the |
| indicator is a large organic molecule that works | | | | indicator is colorless. As H+ ions are further increased |
| somewhat like a “color dye". Whereas most dyes | | | | and pH decreases to pH 4-5, the indicator equilibrium is |
| do not change color with the amount of acid or base | | | | affected and changes to the colorless HIn form. |
| present, there are many molecules, known as acid - | | | | Color changes in molecules can be caused by |
| base indicators, which do respond to a change in the | | | | changes in electron confinement. More confinement |
| hydrogen ion concentration. Most of the indicators are | | | | makes the light absorbed bluer, and less makes it |
| themselves weak acids. The most common indicator is | | | | redder. Here one may ask as to how the electrons |
| found on "litmus" paper. It is red below pH 4.5 and blue | | | | are confined in phenolphthalein? There are three |
| above pH 8.2. | | | | benzene rings in the molecule. Every atom involved in |
| Other commercial pH papers are able to give colors | | | | a double bond has a p orbital which can overlap |
| for every main pH unit. Universal Indicator, which is a | | | | side-to-side with similar atoms next to it. The overlap |
| solution of a mixture of indicators, is able to also | | | | creates a 'pi bond' which allows the electrons in the p |
| provide a full range of colors for the pH scale. A | | | | orbital to be found on either bonded atom. These |
| variety of indicators change color at various pH levels. | | | | electrons can spread like a cloud over any region of |
| A properly selected acid-base indicator can be used to | | | | the molecule that is flat and has alternating double and |
| visually "indicate" the approximate pH of a sample. An | | | | single bonds. Each of the benzene rings is such a |
| indicator is usually some weak organic acid or base | | | | system. |
| dye that changes colors at definite pH values. The | | | | The carbon atom at the center (adjacent to the |
| weak acid form (HIn) will have one color and the weak | | | | oxygen atom) doesn't have a p-orbital available for |
| acid negative ion (In-) will have a different color. | | | | pi-bonding, and it confines the pi electrons to the rings. |
| Phenolphthalein is an indicator of acids (colorless) and | | | | The molecule absorbs in the ultraviolet, and this form |
| bases (pink). Sodium hydroxide is a base, and it was in | | | | of phenolphthalein is colorless. In basic solution, the |
| the pitcher at the beginning, so when added to the | | | | molecule loses one hydrogen ion. Almost instantly, the |
| phenolphthalein in beakers 2 and 4, it turned pink (top | | | | five-sided ring in the center opens and the electronic |
| half of the graphic). In the first beaker, a strange thing | | | | structure around the center carbon changes to a |
| happens in that the pink solution coming out of the | | | | double bond which now does contain pi electrons. The |
| pitcher now changes to colorless. This happens | | | | pi electrons are no longer confined separately to the |
| because the first beaker contains some vinegar or | | | | three benzene rings, but because of the change in |
| acetic acid which neutralizes the NaOH, and changes | | | | geometry around the concerned atoms, the whole |
| the solution from basic to acidic. Under acidic | | | | molecule is now flat and electrons are free to move |
| conditions, the phenolphthalein indicator is colorless. | | | | within the entire molecule. The result of all of these |
| We may use equilibrium principles to explain the color | | | | changes is the change in color to pink. Many other |
| change for phenolphthalein in a discussion. The | | | | indicators behave on the molecular level in a similar |
| simplified reaction is: H+ + OH- ? HOH. As OH- ions | | | | fashion (the details may be different) but the result is a |
| are added, they are consumed by the excess of acid | | | | change in electronic structure along with the removal |
| already in the beaker as expressed in this equation. | | | | of a hydrogen ion from the molecule. Plant pigments in |
| The hydroxide ions keep decreasing and the hydrogen | | | | flowers and leaves also behave in this fashion. |